Anxiety and mood dysregulation are among the most common, often persistent, emotional difficulties in preschool-aged children that cause substantial personal, familial, and economic burden. However, these behaviors are developmentally normative in young children and the level at which such behaviors may be clinically significant is largely unknown. Without established norms identified with developmentally sensitive methods, it is difficult to distinguish mild, transient behaviors from rik for persistent, impairing emotional and behavior problems. Studies have relied solely on parents' judgment to report the subjective frequency of children's behavior and focused primarily on externalizing problems. Information about the daily frequencies of children's anxiety and mood dysregulation, along with familial and physiological correlates of emotional difficulties, wil permit an understanding of the variation in behavior that may increase clinical risk. The primary goal of the present study is to implement a multi-level approach to generate developmentally meaningful distinctions between normative individual differences and patterns of behavior, family environment, and stress physiology that pertain to the development of early-emerging internalizing psychopathology in 3-5-year-old children. To achieve this aim, a novel parent-report daily diary method will be used to generate dimensional phenotypes by assessing the daily frequency of anxiety and mood dysregulation in a large sample (N = 550) of preschool-aged children. To validate the internalizing phenotypes and enhance differentiation of typical from clinically-concerning behavior, the second aim will be to assess children's observed behavior, impairment, parental psychopathology, behavior, and relationship satisfaction, and stressors in a subsample (n = 200) and the third aim will assess acute and chronic stress physiology, including salivary inflammatory markers (C- reactive protein and cytokines) as well as cumulative hypothalamic pituitary axis cortisol activity measured via hair samples. These efforts will advance the NIMH mission to better understand mental illness trajectories. In particular, this work is consistent with Strategic Objective 1.4 to develop valid dimensional phenotypes by assessing biological and behavioral components that clarify the boundaries of mental disorders and delineate the full variation of behavior, and Strategic Objective 2.1 that emphasizes the importance of identifying bio- and behavioral markers of early-emerging psychopathology. Knowledge gained from this proposal will significantly advance the field by 1) providing developmentally sensitive information on the phenomenology of early- emerging psychopathology, 2) advancing the transdiagnostic, dimensional approach by assessing the full variation of anxiety and mood dysregulation, 3) identifying biomarkers that reflect early deviation in emotional development, and 4) inform prevention and intervention efforts for internalizing psychopathology; in addition, consistent with the R15 mechanism, this study will strengthen the PI's instutition's research environment by providing students in-depth experience with multi-method research in developmental psychopathology.